Low orbit satellites are frequently exposed to monatomic oxygen present in the upper limits of the earth's atmosphere. It has been found that certain materials used in satellites are extremely reactive with monatomic oxygen, even in the relatively limited extent to which it is present in the rarefied upper atmosphere. These materials are significantly degraded or destroyed through the process of chemical oxidation resulting from exposure to this substance. Since there is little correlation to the reaction of the material with diatomic oxygen molecules, O.sub.2, the state in which oxygen is normally found on the surface of the earth, it is difficult to predict the reaction potential of a material with monatomic oxygen. Therefore, empirical testing provides the only reliable basis for determining whether a material is sufficiently resistant to reaction with monatomic oxygen for use in satellites. To carry out such tests, a source of monatomic oxygen is required.
Although normally used to produce positively charged ions by removing one or more valence electrons from an atom, a plasma generator may also be used to dissociate diatomic oxygen molecules into monatomic oxygen. Certain types of plasma generators use a DC field or an RF induction coil to generate ionized particles, but are not suitable for producing monatomic oxygen for testing material specimens, because the heat generated by either of these methods does not properly simulate the desired low temperature conditions of outer space. However, another type of plasma generator produces a relatively low temperature plasma by ionizing atoms as they pass between two spaced-apart electrodes that are excited with an RF signal. Even using this method, it is preferable to produce the monatomic oxygen in a separate section of the plasma generator, so that it cools slightly before entering a test section in which the material under test is disposed.
In the conventional approach to generating a low temperature plasma with this method, one of the electrodes is grounded and an RF signal referenced to ground potential is applied to the other electrode. In attempting to use apparatus thus constructed, it was found that substantial RF energy was radiated by the plasma generator, requiring that it be fully shielded to prevent radiation levels in its vicinity from exceeding safe limits. Even with this shielding, radiation emanating from the unit interfered with thermocouples and other nearby laboratory equipment used in monitoring the test section. In addition, specimens and a specimen holder disposed in the test section of the device were subjected to heating caused by a plasma glow discharge current flowing from the energized (nongrounded) electrode to a grounded screen disposed in the test section. The grounded screen was installed to protect a vacuum pump attached to the device from the glow discharge current. These problems made it difficult, if not impossible, to use the low temperature plasma generator for its intended purpose.
Accordingly, it is an object of the present invention to substantially reduce the RF radiation from a low temperature plasma generator, and to eliminate heating of a specimen due to a glow discharge current. These and other objects and advantages of the present invention will be apparent from the attached drawings and from the Description of the Preferred Embodiment that follows.